Our Love for Dogs May Be Coded in Our DNA

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I have a friend, Barbara, who has been involved with dogs as a breeder and as a competitor in conformation and obedience trials all of her life. The interesting thing is that her parents were also very much involved in dog breeding and competition, and her daughter, Cindy, is similarly active in dog-related activities.

Compare her to a colleague of mine, Paul, who has never owned a dog, nor had any desire to do so. Paul's parents also never had a dog as a family pet, and his son, Derek, who is now already married and in his mid-20s, does not have a dog in his family and has no desire to get one in the future.

As a psychologist, I have always been interested in which factors determine certain patterns of behavior, especially when those patterns seem to be passed on to subsequent generations in the same family. When it comes to positive or negative feelings toward dogs and dog ownership, I have always presumed that this is determined partly by culture — for example, certain religions may support or abhor interactions with dogs — and partly by an individual's personal history. This idea has been supported by the fact that previous research has indicated that exposure to dogs during childhood is associated with more positive attitudes toward dogs and an increased likelihood of dog ownership in adulthood.

However, there is an alternative.

A team of Swedish researchers headed by Tove Fall, a Professor of Molecular Epidemiology at the Department of Medical Sciences and the Science for Life Laboratory at Uppsala University, was aware of data showing that certain personality characteristics were more common in dog owners than in owners of cats or non-pet owners. (Click here for an example.) Since there is already a lot of evidence showing that there are genetic influences on personality characteristics, it is a natural next step to wonder if there might be a genetic factor that plays a role in determining whether a person will be attracted to dogs and want to own one.

This research group had some incredibly useful tools which allowed them to study this question. First, they had access to the Swedish Twin Registry, founded in the late 1950s, which contains information on all twins born in Sweden. Second, information on dog ownership has been available since January 1, 2001 when two dog registries were established, one at the Swedish Board of Agriculture and the other at the Swedish Kennel Club. There is a legal requirement that all dogs in Sweden must be registered.

The reason why the twin registry was so important is because the most powerful way to answer the question, "Is this particular factor influenced by genetics?" is to look at twins. There are two types of twins, identical and fraternal. Identical twins arise when a single egg is fertilized by a single sperm cell to form what is technically called a "zygote." As that zygote develops, usually within the first or second cell division, it splits apart to form two separate individuals. The magic of this is that these two individuals, since they come from the same sperm and ovum, have identical genetic makeup. Thus the resulting individuals will be of the same sex, will look the same, and share every characteristic that is genetically determined. Because these twins come from the same zygote they are technically called "monozygotic twins" which is typically abbreviated as "MZ twins."

Contrast these identical twins to fraternal twins. Fraternal twins come about when two egg cells are fertilized by two different sperm cells and develop in the same womb. In this case, the zygotes only share the same amount of genetic material that any pair of brothers or sisters might. They might not look the same, and might not even be the same sex. Since the process began with two zygotes, these are technically called "dizygotic twins" and typically abbreviated as "DZ twins."

Regardless of whether we are dealing with identical or fraternal twins, both kinds have very similar personal histories since they grow up in the same family and share the same environment, culture, and parenting influences. So, if we compare the similarities in the behaviors of the two types of twins, and we find greater similarities between the identical twins (with their matching genetic makeup) than between the fraternal twins (who only share about half of their genes), this would confirm that we are looking at something which has a genetic component.

These Swedish researchers looked at 35,035 pairs of twins, determined whether they were identical (MZ) or fraternal (DZ), and then determined whether each twin had gone on to become a dog owner. If there is a genetic factor then the "concordance rate" should be higher for the monozygotic twins. Concordance would be shown when both twins owned dogs, or both twins did not own dogs, while a lack of concordance would be shown by the fact that one twin owned a dog and the other did not. The researchers concluded that "MZ twins had higher concordance rates and tetrachoric correlations (0.58 for females and 0.52 for males) than DZ twins (0.35 for females and 0.30 for males), in line with presence of genetic effects."

Although these results seem to confirm that there is a genetic factor associated with dog ownership, it doesn't tell us how much influence genetics has. For this, the researchers had to do a lot of statistical analysis involving structural equations, and they came up with the estimate that the size of the inherited component influencing whether we are impelled to own a dog or not is 57 percent for females and 51 percent for males. In other words, approximately half of the psychological pressure that we feel to own or not to own a dog is influenced by our DNA, with the other half influenced by environmental factors such as our personal histories or culture.

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It may appear a bit strange and perhaps unbelievable to suggest that our attitudes toward specific animal species could have a genetic component. However, there are other examples. Among humans there is an almost universally negative and fearful response to spiders and snakes, and this is clearly genetically determined.

While this kind of twin study can't tell us exactly which genes are involved, it does demonstrate for the first time that genetics and environment play about equal roles in determining dog ownership. As lead author Tove Fall commented, "We were surprised to see that a person's genetic makeup appears to be a significant influence in whether they own a dog." She suggested that some genetically determined personality factors might underlie this as she speculated, "Perhaps some people have a higher innate propensity to care for a pet than others."

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